Radio frequency measurements

ABSTRACT

There is disclosed a method for determining the absolute response of a signal strength meter, or the absolute response of a device under test with the use of a previously calibrated signal strength meter. In the former case, a continuous wave signal of predetermined amplitude and frequency is applied to the input of the meter while it is tuned to the frequency of the continuous wave signal. A white noise signal is then applied to the input of the meter while it is tuned to the same frequency, and the level of the white noise signal is adjusted so that the meter reading is the same as the reading taken for the continuous wave signal. Thereafter, as the signal strength meter is tuned throughout the frequency range of interest, the absolute response of the signal strength meter can be determined by observing the meter reading even though the bandwidth of the filter of the meter may not be known. In a similar manner, a previously calibrated meter can be used to measure the absolute response of a device under test to whose input there are applied the white noise and continuous wave signals, and whose output is connected to the meter.

United States Patent Sadel [54] RADIO FREQUENCY MEASUREMENTS [76]Inventor: Hans Sadel, 299 Park Avenue,

Weehawken, NJ. 07097 [22] Filed: June 1, 1971 [21] Appl. No.: 148,599

[52] US. Cl. ..324/57 N, 331/78 [51] Int. Cl. ..G01r 17/04 [58] Field ofSearch ..324/57 N; 331/78 [56] References Cited UNITED STATES PATENTS3,281,711 10/1966 Kees et al. ..33l/78- 1,914,414 6/1933 Fairchild..324/57 N 2,773,186 12/1956 Herrmann, Jr. ....324/57 N 2,883,616 4/1959Sabaroff ..324/57 N 2,989,700 6/1961 ..324/l03 3,102,231 8/1963 Wolf..324/57 N OTHER PUBLICATIONS Electronics, Jan.

ABSTRACT There is disclosed a method for determining the absoluteresponse of a signal strength meter, or the absolute response of adevice under test with the use of a previously calibrated signalstrength meter. In the former case, a continuous wave signal ofpredetermined amplitude and frequency is applied to the input of themeter while it is tuned to the frequency of the continuous wave signal.A white noise signal is then applied to the input of the meter while itis tuned to the same frequency, and the level of the white noise signalis adjusted so that the meter reading is the same as the reading takenfor the continuous wave signal. Thereafter, as the signal strength meteris tuned throughout the frequency range of interest, the absoluteresponse of the signal strength meter can be determined by observing themeter reading even though the bandwidth of the filter of the meter maynot be known. In a similar manner, a previously calibrated meter can beused to measure the absolute response of a device under test to whoseinput there are applied the white noise and continuous wave signals, andwhose output is connected to the meter.

4 Claims, 2 Drawing Figures l -3 cw LEVEL MoNIToR CURRENT AND OPERATINGMONITOR VOLTAGE MONITOR II VOLTAGE REGULATOR 2 8 8 O F NOISE DIODECURRENT NoIsE l CON oL DIoDE 12/ wIDE BAND AMPLIFIER 6% LEVEL O CONTROLh-MW-l \-,9

3 QEDMBINING NETWORK PAIEIITEII 1 1915 3,731 186 7 M f l p f u I 3 TBATTERY CHARGER CURRENT cw LEvEL MONITOR AND OPERATING MONITOR vOLTAGEMONITOR /II vOLTAGE T 2 REGULATOR 68) OFF 8 NOIsE DIODE I CURRENT ISE ICW 1 NOIsE CONT OL DIODE I I L 12 6 wIDE BAND 7 "EE'EE 6 LEVEL AOscILLATOR I [2 0 \4 I VT/9 HARMONIG A L 5 3 v FILTER QGOMBINING 27ggg'g NETWORK ATTENUATOR l l baF OUTPUT 2! I 1L/-Z3 RETURN LOSS 2 L/ lFAMIEE $25 30 34 w [I3 Lu. l METER 5 Q5 1 I 3/ 2 AMPLIFIER y .L 37 T-AMN\--I I l k l I E; WV-IF NOISE DIODE A CURRENT 36 cONTRO II- I TNOISE I GURRENT D'ODE MONITOR INvENTOR I VOLTAGE REGULATOR RADIOFREQUENCY MEASUREMENTS This invention consists of a'method and apparatusfor RF. measurements, wherein several signals are produced, including awhite noise signal covering a range of frequencies, and a continuouswave (CW) signal ofa frequency within this range; and wherein the CW.signal is applied as a reference level for the white noise signal, andreference signals are derived from the white noise signal for at least aportion of its spectrum.

The conventional method of making basic measurements on communicationsand other electronic equipment, has been in the use ofa singlefrequency, tunable generator and a wide-band receiver or indicator. Inthe television industry, however, it became necessary to accuratelymeasure the level of a narrow-band signal in the presence of a widespectrum of similar signals. This led to the development of portable,tunable, narrowband signal strength meters, also called field strengthmeters or R.F. voltmeters. With this type of meter being widely acceptedand present in the television and communications industry, it has alsobecome practical to use a wide-band signal generator for manymeasurements.

Thus, for example, a sweep frequency generator is used in conjunctionwith a tunable, narrow-band signal strength meter to measure theresponse of an amplifier, a filter or other components, especially as afunction of frequency; however, the low sweep frequency of most commonlyavailable sweep frequency generators, may cause the needle of the signalstrength meter to vibrate, thus making accurate readings difficult.Furthermore, the detector circuit of the signal strength meter may havea narrow band width and long time constants, causing it to be incapableof developing the full signal amplitude when the signal generator isadjusted to sweep a wide band; the signal will sweep through the passband too quickly; and generators not having a constant sweep ratethroughout their frequency range, will cause a signal strength meter offixed bandwidth to indicate the signal amplitude with varying accuracythroughout the frequency range. Since, in practice, it is frequentlydesirable to set the sweep width to sweep the complete VHF TV spectrum(54 to 216 MHZ), it has been found that the sweep rate at the highfrequency end of the spectrum is faster than at the low frequency end,causing the signal to remain in the pass band of the meter a shortertime than when turned to the low end. This results in a lower indicationon the signal strength meter when it is tuned to the high frequency endthan when it is tuned to the low end of the spectrum.

Generally, most of the available wide. band generators have one or'moreof the following disadvantages when used in conjunction with ordinarysignal strength meters:

l. The signal level is very low as, for example, in noise generatorsintended for the measuring of noise figures. i

2. ln sweep frequency generators, the signal strength meter may see, ineffect, a pulsed signal, causing needle vibration.

3. The signal strength meter may also see, in effect, a signal whichvaries in amplitude through the spectrum, as would be caused by anon-linear sweep rate.

4. No convenient method of making an absolute level determination isprovided.

5. The generator is not truly portable to the extent of being light inweight, small in size, and having a selfcontained power supply.

One of the objects of the invention is to reduce or eliminate thesedrawbacks by providing a dual-type of signal generator which selectivelyemits either white noise or a CW signal, combined with a radio frequencybridge and an attenuator.

Another object of the invention is to calibrate signal strength meters,and further, in conjunction with a signal strength meter, to measureR.F. parameters of many active devices such as amplifiers and mixers,and of passive devices such as attenuators, filters, cables and othertransmission lines, splitters and terminations.

These and other objects of the invention will be more fully apparentfrom the drawing annexed herewith, in which FIG. 1 shows a circuitdiagram embodying certain principles of the invention.

FIG. 2 exemplifies a portion of FIG. 1, also in a diagramatical fashion.

The circuitry of FIG. 1 is adapted to perform a number of differentmeasurement operations.

An important application is in the calibration of signal strengthmeters.

Until now, signal strength meters were calibrated for response bycarefully tuning both a calibrated signal generator and the signalstrength meter to each frequency of interest, often 10 to 20frequencies, and noting the error in the deflection of the signalstrength meter. Thus, point by point, the frequency response of themeter was laboriously determined.

In accordance with one embodiment of the invention, a white noisesignal, offering the convenience of a signal of constant amplitude andcontinuity over the whole spectrum, is calibrated with a continuous wave.signal. If a white noise signal were used alone to calibrate anarrow-band signal strength meter, it would be necessary to know theabsolute level of the white noise signal, the detector efficiency of thesignal strength meter for white noise as well as for a CW signal, andthe bandwidth of the signal strength meter. These three parametersare'very difficult to measure, especially in the VHF and UHF ranges. I

In accordance with the invention, an absolute reference level isestablished by means of an internal CW signal, after which the signalstrength meter is tuned to each frequency of interest.

In practice, and in connection with the circuitry of FIG. 1, a whitenoise signal derived from the output of a white noise generator, such asa diode as schematically indicated in FIG. 1, at 1, is carefullyadjusted by current control 12 so that, over the entire frequency rangeof interest, as observed on current monitor 3, there will be less than,say, 1 dB variation above or below a predetermined value, when measuredwith a signal strength meter having a bandwidth of 0.5 MHZ. The whitenoise signal output level can then be adjusted by the operator through arange sufficient to insure that signal strength meters having bandwidthsbetween 0.1 and 2.0 MHz, can be brought to the same deflection as thatcaused by a l millivolt CW signal applied to the input.

According to FIG. 1, the CW signal is the output of fixed frequencyoscillator 4, the frequency of which might be chosen to be anywhere inthe frequency range of interest which is of convenience to the user. Forinstance, it has been found to be practicalfor the television industryto use as a reference frequency, a frequency between channels 4 and 5.Oscillator 4 may be crystal controlled. In the final alignment of thecircuit of FIG. 1, the output level of the cw signal is precisely set,providing a reference signal level which is its principal function. Forthis purpose switch 5 is put into position I and level control 6 isadjusted while level monitor 7 is being observed. At the same time,switch 8 is in position I connecting battery 9 which may be chargeablefrom charger l over voltage regulator 1 1 to CW oscillator 4.

With switch in position II, the white noise circuit of FIG. 2 is usedfor calibration or any other measurements, in accordance with theinvention. In this position of switch 5, voltage level monitor 7 andvoltage regulator 11 are operative, under control of noise currentcontrol 2, to energize noise diode 1. In position I of switch 5,oscillator 4 is in use, and its output is monitored on 7, which islocated on the front panel of the device supporting the circuitry ofFIG. 1, and which is to provide a continuous, accurate visual indicationof the output level for the operator. For the television industry acommon reference level is l millivolt across 75 ohms.

In position ll of switch 5 noise generator 1 is in use,

a and in accordance with the invention, a signal strength meter to becalibrated, as schematically indicated at 13, is connected to RF outputterminal 16. On the other end, noise diode 1 is connected over wide-bandamplifier 17 which may contain level control 18 to combining network 19,which at one side is connected over harmonic filter 20 to level control6 ofCW oscillator 4. On the other side, combining network 19 connectsthrough switch 21 and attenuator 22 to RF output terminal 16.

Now, selectively, and in accordance with the invention, either a CWsignal of predetermined reference level, or a white noise signal ofadjustable level, is connected to the signal strength meter to becalibrated; and after having established an absolute reference with theinternal CW generator 4, in position I of switch 5, it only becomesnecessary, in position II of switch 5, to adjust the level of the whitenoise signal at the CW frequency by varying level control 18, and thento tune the signal strength meter to each frequency of interest. In thiscase, the white noise generator 1, emitting a signal of constantamplitude throughout the frequency range of interest, will result in thesame constant deflection of signal strength meter 13 throughout itsfrequency range, if the response of that signal strength meter is flat,even while the meter is being tuned. Since the deflection is constant,the electrical and mechanical time constants of the signal'strengthmeter, are much less of a problem than is usually encountered when aconventional method of measurement is used.

With signal strength meter 13 connected to terminal 16, and receiving inposition ll of switch 5, .the white noise signal derived from generator1 will indicate a reading which is dependent on its receiver bandwidth:Meters having a larger bandwidth will accept more of the white noisesignal spectrum, and thus indicate a stronger signal. It may be expectedthat signal strength meters, even of the same make and model, havesignificant differences in bandwidth.

In accordance with the invention, this problem is dealt with in thefollowing manner:

. First, in position I of switch 5, the signal strength meter, or anyother measurement instrument to be evaluated, is switched to the CWoutput signal derived from generator 4, and to which all signal strengthmeters respond alike when tuned to it, regardless of their bandwidth.This is because the CW signal is extremely narrow as compared to thebandwidth of any signal strength meter, and all of the CW signal energyis received by the meter. Since in accordance with the invention, the CWsignal output level has been accurately set as explained previously, thesignal strength meter being calibrated must be adjusted to indicate thisvalue, or its error noted.

Second, in position II of switch 5, the signal strength meter, or anyother instrument to be calibrated, is switched to the white noise signalderived from generator 1, while the signal strength meter undercalibration is left tuned to the CW signal frequency.

Now, by using the variable output control 18 of the noise signal, andstep attenuator 22, the reading on signal strength meter 13 is adjustedto the exact reading observed with the CW signal. While eachmanufacturers signal strength meter will have a different bandwidthwhich will cause different output indieations, this level adjustmenteliminates the effect of the meters bandwidth on the indication. Thewhite noise signal level is simply adjusted to produce exactly the sameindication which the CW signal has produced. Now, as the signal strengthmeter 13 is tuned through its frequency range, this meter, without anyfurther adjustment, will continuously indicate a response to the whitenoise signal, and since the white noise signal output is essentially thesame at all points throughout the frequency range of interest, thedeviation appearing on the signal'strength meter, from the valuepreviously determined as the reference level at the CW referencefrequency, constitutes directly the frequency response of the signalstrength meter under calibration, provided the bandwidth of the meter isthe same throughout its tuning range; and as quickly as the meter can betuned to other frequencies, data can be acquired for plotting a completefrequency response characteristic of the meter.

The particular frequency of the greatest, or of the least, responsewithin the frequency range of interest, is quickly found by merelyturning the signal strength meter to the frequency of the greatest, orof the least, deflection. At the same time, the CW signal which has anaccurately known absolute level and is completely received by all signalstrength meters, is instantly and always available from the circuitaccording to my invention, permitting an absolute level reference pointto be established in the frequency range of the signal strength meter.Thus, without cable changing or rearranging any external circuitry, byoperating switch 5,

level control 18 and adjustment of attenuator 22, the complete'frequencyresponse of the meter under test is very quickly determined.

While in this part of the application of the invention, the CW and whitenoise signals have been applied over certain common elements such ascombining resistance network 19, switch 21 and step attenuator 22, to acommon RF output terminal, the invention is not limited to theparticular circuit separations and connections shown and described, norto the specific types of circuit elements involved; but it may also beapplied in another appropriate way or manner, without departing from thescope of the disclosure.

One of the most frequent evaluations required in the communications andtelevision industry is the measurement of the frequency response ofcomponents such as the gain or loss of amplifier, filters, cables andother transmission lines.

Until now, the frequency response of narrowand wide-band amplifiers,filters, cables and other components in the VHF and UHF range, wasdetermined by using various methods and apparatus, most commonly thefollowing:

I. A manually tuned CW generator and a manually tuned receiver requiringa laborious tuning procedure.

2. A sweep frequency generator, a detector, and a triggered orsynchronized-sweep oscilloscope, from which it is usually difficult tomake accurate visual readings to permit transfer to a permanent record.

In this particular application of the invention, a system such asoutlined with respect to FIG. 1 is applied in connection with anordinary field strength meter, to determine the response of anamplifier, filter, or other frequency-dependent circuit component. Inthis case, too, after having selected a suitable reference level asexplained previously the signal strength meter such as shown in FIG. 1at 13, is tuned through the frequency range ofinterest; at thefrequencies ofinterest, the operator stops and reads the meter, and theproblems and-solutions are similar to those discussed above inconnection with the calibration of signal strength meters, in general.Here, too, the white noise signal generator 1, emitting a signal ofconstant. amplitude throughout the frequency range concerned, will causethe output spectrum of the amplifier or filter, schematically indicatedin FIG. 1 at 23, to assume the shape of the frequency responsecharacteristics curve of the amplifier or filter concerned.

More specifically, and in accordance with the procedure previouslyoutlined for meter calibration, the absolute level of the white noisesignal atthe input of unit 23, which in this case is terminal 16, isadjusted in accordance with the bandwidth of the signal strength meter13 so as to give a particular reference output at one frequency such asO dBmV. Readings of the meter 13 at all other frequencies, throughoutthe frequency range of interest, will indicate not only the relativefrequency response of unit 23 under test, but also its gain or loss.

Since the white noise signal is of constant amplitude throughout thefrequency range of interest, the signal strength meter will continuouslyindicate the gain or loss of the amplifier, or filter, even while thesignal strength meter is being tuned. Thus, the meters electrical andmechanical time constants pose much less ofa problem than is usuallyencountered in the use of conventional equipment and methods.

Furthermore, any error due to mistuning of the signal strength meter,caused to a CW signal generator, is inconsequential, if not outrightexcluded; so is the problem of the drift of either the CW generator orthe signal strength meter, or both.

In the practical application of this part of the invention, first, theamplifier or filter to be evaluated is bypassed; and the white noisesignal, at terminal 16, is directly connected to the field strengthmeter indicated at 13, at any convenient frequency setting of the meter.The white noise signal is then adjusted, by control 16 and attenuator22, for a convenient reading of reference level on meter 13, such as aunity calibration mark on a voltage scale, or a zero mark on a decibelscale. Next, unit 23 to be tested, is connected between terminal 16 andmeter 13 as schematically indicated at 23. The meter now directlyindicates the gain or loss of unit 23; and as quickly as the signalstrength meter can be tuned to other frequencies, the response level maybe observed, or data can be acquired for plotting a frequency responsecharacteristic showing gain or loss through the pass band, andattenuation in the stop band.

High and low values of the frequency response characteristic for theunit under test, and their frequencies, are quickly found, merely bytuning the signal strength meter to maximum and minimum deflections.

Another and rather important application embodying the invention is inthe field of return loss measurements.

Until now, the condition for matching a load to a source was usuallymeasured in terms of VSWR, especially if the precise nature of the loadwas of interest, for example, from the point of view of the designengineer. In the case, however, that only a qualitative, instead of aquantitative, measure of matching condition is required, it oftenbecomes expedient to measure indirectly that power which a mismatchedload does not absorb, as compared to that power which is a perfectlymatched load would absorb from the same source. (This unabsorbed poweris spoken of as reflected power. When compared directly to that powerwhich a perfectly matched load would absorb, the ratio is called returnloss).

In the past, return loss measurements have been made with the use ofvarious methods and apparatus, most commonly the following:

I. A manually tuned CW signal generator, a return loss bridge, adetector, and a calibrated indicator, usually requiring the signalgenerators output level to be monitored and readjusted to compensate forthe change of output level at different frequencies.

2. A sweep frequency signal generator, a return loss bridge, a detectorand a triggered or synchronizedsweep oscilloscope, from which it isusually difficult to make accurate visual readings for transfer to apermanent record. Linear oscilloscope displays have a limited dynamicrange of adequate resolution, and logarithmic displays have inadequateresolution throughout the dynamic range, on most equipment.

In both of the above methods, the cost of equipment which would permitacceptable accuracy of measurement in the VHF range, is quite high; theequipment is frequently not conveniently portable with respect to size,weight and power source, and often the circuit arrangements are morecomplex than required for the practical use of these devices.

In accordance with a further embodiment of this invention, alsoillustrated in FIG. 1, which may or may not be used together with theprevious or any other embodiments, the return loss of electricalcomponents is measured, having a nominal impedance which is comparableto that of the impedance bridge in this invention. I

An example of such a bridge is indicated in FIG. 1 at 24, and it can bedesigned for any impedancesuch as 50, 70, 75 and 90 ohms. Electricalcomponents which can be measured with this embodiment of the inventionare: Attenuators, input and output impedances of amplifiers, filters andmixers, resistors, terminated cables or other transmission lines,terminations, etc.

In order to determine the return loss of the unit schematicallyindicated in FIG. 1 at 25, and connected to return loss terminal 26,switch 21 is put into position 1 1 connecting the output of resistancenetwork 19 over balun 27 to diagonal junction points of bridge 24, whilea diagonal point of the bridge is connected over terminal 26 to unit 25.Another junction point of bridge 24 is connected, in position 11 ofswitch 21, over step attenuator 22 and terminal 16 to an ordinary signalstrength meter, such as schematically indicated in FIG. 1 at 13.

The return loss of unit 25 is now determined in accordance with theinvention, by tuning signal strength meter 13 through the frequencyrange of interest, after having established a reference level, stoppingat those frequencies desired, and reading the signal strength meter asexplained in the previous applications of the invention. The white noisegenerator 1, emitting a signal of fixed and constant amplitude to returnloss bridge 24, throughout the frequency range concerned, will result ina signal level reaching meter 13, which is directly equal to the powerreflected from the unit 25, at all frequencies throughout the frequencyrange of interest. Signal strength meter 13 will continuously indicatethe reflected power, even while being tuned. Thus, the meters electricaland mechanical time constants are much less of a problem than is usuallyencountered in the use of conventional equipment and methods. As in theprevious embodiments of the invention, any error due to mistuning of themeter on a CW signal frequency is eliminated, as is the problem offrequency drift of either the CW generator or signal strength meter, orboth.

In the practical application of this embodiment of the invention, thefollowing procedure may be applied:

First, before connecting the unit to be tested to terminal 26, themeasurement system such as exemplified in FIG. 1, should be prepared forreturn loss measurement. With no termination on return loss jack 26, andat any convenient frequency setting of meter 13 connected to terminal16, the level of white noise signal generator 1 is adjusted for aconvenient reference level on the signal strength meter 13, such as aunity calibration mark on a voltage scale, or a zero mark on a decibelscale. With no power being absorbed in a load, the return loss is nowone, or zero dB. Now the unit to be measured, such as schematicallyindicated in FIG. 1 at 25, is connected to jack 26, signal strengthmeter 13 is tuned to the frequency desired. The signal to the meterrepresents the return loss of unit 25. For in stance, if the meterreading drops to one tenth of the reference value, the return loss on avoltage scale would be (converting relative voltage to relative power)0. l 0.01, while on a decibel scale one should read 20 dB; and asquickly as meter 13 can be turned to other frequencies, the return losscan be observed, or data be acquired for plotting a return losscharacteristic over the entire frequency range of interest. Maxima andminima of the return loss characteristic for the unit under test, andtheir frequencies, are quickly found, merely by tuning meter 13 tomaxima and minima of deflection. Thus, for example, to determine howclosely unit 25 approaches or exceeds a certain return lossspecification, the operation of only a single control member will permitthe zeroing-in on this critical information.

A further embodiment of the invention permits the measurement of lengthand the location of faults in cables or other transmission lines. Thus,for example, cable length, and the location of a fault in a coaxialcable, can be measured with substantially no additional circuitry orcost being necessary in a system, such as exemplified in FIG. 1. Thenature of the signal derived from this circuitry permits this type ofmeasurement with any ordinary signal strength meter covering an adequatefrequency range. In conjunction with such a signal strength meter asschematically indicated in FIG. 1 at 13, it becomes necessary only totune the meter to two frequencies of minimum response, and to note thesefrequencies accurately. White noise signal generator 1, emitting asignal of constant amplitude throughout the frequency range of interest,will cause a constant deflection of the signal strength meter, if it isconnected to the center connector of a so-called TEE connectorschematically indicated in FIG. 1 at 28, which in turn is connected toterminal 16. TEE connector 28 is a T-shaped junction of threeconnectors, one of which serves to connect to terminal 16, while the twoother ones are connected to signal strength meter 13 and cable 29,respectively, the latter being the cable to be tested with respect tolength or location of fault. Withcable 29 under test being so connected,the cable impedance presented to TEE junction 28 will vary throughoutthe spectrum of white noise signal generator l; and for a cable 29,which is open internally, or at the far end, at all those frequencies atwhich this cable has the length of an odd number of electricalquarterwave lengths, the cable impedance presented to TEE junction 28will be extremely low. Similarly, for a cable which is shortedinternally, or at the far end, at all those frequencies at which thecable is an even number of electrical quarter-wave lengths long, thecable impedance presented to TEE junction 28 will also be extremely low.This low impedance present at TEE junction 28 greatly reduces the signalto signal strength meter 13. For both the open ended and the shortedcondition, the frequency difference between any two consecutivefrequencies at which the cable under test presents a low impedance toTEE junction 28, is exactly the frequency at which the cable under testis an electrical half-wavelength long.

In the practical application of this embodiment of the invention, and asan example, the following procedure may be adopted:

First, terminal 16 and a signal strength meter are connected to theterminals of the TEE junction such as illustrated in FIG. 1 at 28. Atany convenient frequency setting of the meter, which again isexemplified by box 13, the white noise signal level of generator l isadjusted for a nearly full scale deflection on the signal strength meter13. Then, the cable length to be measured is connected to TEE junction28, as exemplified by 29. Thereafter, meter 13 is turned to either endof its tuning range and from there it is gradually tuned toward themidrange frequencies until a signal minimum is found. The frequency ofthis minimum is carefully noted. The tuning is then continued in thesame direction until the next frequency minimum is found, and itsfrequency is also accurately noted. The frequency difference betweenthese two noted frequencies represents the electrical half-wave lengthfrequency ofthe cable 29 under test.

The actualcable length or the distance to a short or an open in thecable can be calculated from the following equation:

where fx/z is the electrical half-wave length frequency in MHz, and ethe effective dielectric constant of the cable under test.

In accordance with a further feature of the invention, improved accuracymay be obtained in the following manner:

Instead of stopping at the next consecutive signal minimum, the tuningof meter 13 is continued, say to the n" consecutive signal minimum, andits frequency is accurately noted. The electrical half-wave length ofthe cable under test is now represented by the difference between thetwo noted frequencies, divided by the factor n.

FIG. 2 exemplifies part of the circuitry shown in FIG. 1, in greaterdetail, and with certain specific improvements applied to the generationof the white noise signal, in accordance with certain other features ofthe invention.

In this circuitry, the wide band amplifier which amplifies the noisefrom the white noise generator, is designated at 30, and it is designedto permit the adjustment of the noise signal amplitude throughout thedesired frequency spectrum. Amplifier 30 has five resistance-capacitancecoupled transistor stages, schematically indicated at 31 to 35, with thecollector circuits having essentially flat response throughout thedesired frequency range. Further flattening of the noise spectrum isaccomplished by means of adjustable, low impedance,resistance-capacitance networks. in the transistors emitter circuits.Thus, the more troublesome resonant circuits are not used to obtain thedesired frequency response. With the frequencydeterminingcharacteristics of the emitter circuits being spaced across the desiredfrequency range, it is possible to achieve a white noise spectrum havingless than plus or minus one dB variation throughout the desiredfrequency range. The white noise signal generator consists essentiallyof a noise diode schematically indicated in FIG. 2 at 36, preferably ofthe Zener type which, in accordance with a specific feature of theinvention, is directly coupled to the base of amplifier transistor 37 sothat the total diode biasing current flows into the base circuit ofamplifier transistor 37. Measurements made with the noise diode beingdirectly coupled to the base of the amplifier transistor, indicate thetemperature coefficient of the output level to be less than 1-0.002dB/F, from 74 to 30F, and +0.01 dB/F from meters feet.

74 to 104F. For the overall temperature range of 30 to 104F the averagetemperature coefficient is +0.004 dB/ F. Measurements made withcapacitively coupling of the noise diode to the base of the amplifiertransistor, results in the output having a temperature coefficient of0.2 dB/F over the temperature range from 30 to 104F. Thus, when directlycoupled, the temperature characteristic of the noise diode nearlycancels the temperature characteristic of the amplifier.

While the invention has been described and illustrated by way of anumber of examples, applications,

and embodiments, and also in the form of systems and arrangements ofcircuits and circuit elements, the invention is not limited to thecircuits, circuit elements and connections, nor to the measurements andmethods therefor, shown or described, but may be applied within theskill of anyone familiar with the art of radio frequency measurements,without departing from the scope of this disclosure.

Iclaim: I

1. A method for determining the absolute response of a device under testthroughout a frequency range of interest by using a tunable instrumentfor measuring and indicating the power in a fixed bandwidth whose centerfrequency may be varied, comprising the steps of:

a. tuning said instrument to a predetermined frequency,

b. utilizing the instrument as tuned in step (a) to measure the responseof the device under test to a continuous wave signal of predeterminedamplitude whose frequency is the same as said predtermined frequency,and

' c. varying the center frequency of said instrument over the frequencyrange of interest in order to measure the response of the device undertest to a white noise signal of uniform amplitude throughout saidfrequency range of interest, after first adjusting the amplitude of saidwhite noise signal so that the measurement indicated by said instrumentwhile it is tuned to said predetermined frequency is the same as themeasurement indicated in step (b).

2. A method for determining the absolute response of a tunableinstrument throughout a frequency range of interest, said instrumentbeing operative to measure and indicate the power in a fixed bandwidthwhose center frequency may be varied, comprising the steps of:

a. tuning said instrument to a predetermined frequency,

b. measuring the response of the instrument as tuned in step (a) to acontinuous wave signal of predetermined amplitude whose frequency is thesame as said predetermined frequency, and

c. varying the center frequency of said instrument over the frequencyrange of interest in order to determine its response to a white noisesignal of uniform amplitude throughout said frequency range of interest,after first adjusting the amplitude of said white noise signal sothatthe measurement indicated by said instrument while it is tuned tosaid predetermined frequency is the same as the measurement indicated instep (b).

3. A system for determining the absolute response of a device under testthroughout a frequency range of interest comprising a tunable instrumentfor measuring and indicating the power in a fixed bandwidth whose centerfrequency may be varied, and signal generating means, the deviceunder-test being connected between said signal generating means and saidinstrument such that said instrument measures and indicates the responseof said device under test to the signal furnished thereto by said signalgenerating means, said signal generating means including means forgenerating a continuous wave signal of predetermined amplitude andfrequency, white noise signal generating means, means for selectivelyapplying one of said continuous wave and whitenoise signals to saiddevice under test, and means for adjusting the amplitude of said whitenoise signal such that said, instrument indicates the vsame measurementwhile it is tuned to said predetermined frequency when each of saidcontinuous wave' and white noise signals is applied to said device undertest.

4. A system for determining the absolute response of a tunableinstrument throughout a frequency range of interest, said instrumentbeing operative to measure and indicate the power in'a fixed bandwidthwhose center frequency may be varied, comprising signal generating meansfor connection to said instrument such that said instrument indicatesits response to the signal applied thereto by said signal generatingmeans, said signal generating means including means for generating acontinuous wave signal of predetermined amplitude and frequency, whitenoise signal generating means, means for selectively applying one ofsaid continuous wave and white noise signals to said instrument, andmeans for adjusting the amplitude of said white noise signal such thatsaid instrument indicates the same measurement while it is tuned to saidpredetermined frequency when each of said continuous wave and whitenoise signals is applied thereto.

1. A method for determining the absolute response of a device under testthroughout a frequency range of interest by using a tunable instrumentfor measuring and indicating the power in a fixed bandwidth whose centerfrequency may be varied, comprising the steps of: a. tuning saidinstrument to a predetermined frequency, b. utilizing the instrument astuned in step (a) to measure the response of the device under test to acontinuous wave signal of predetermined amplitude whose frequency is thesame as said predtermined frequency, and c. varying the center frequencyof said instrument over the frequency range of interest in order tomeasure the response of the device under test to a white noise signal ofuniform amplitude throughout said frequency range of interest, afterfirst adjusting the amplitude of said white noise signal so that themeasurement indicated by said instrument while it is tuned to saidpredetermined frequency is the same as the measurement indicated in step(b).
 2. A method for determining the absolute response of a tunableinstrument throughout a frequency range of interest, said instrumentbeing operative to measure and indicate the power in a fixed bandwidthwhose center frequency may be varied, comprising the steps of: a. tuningsaid instrument to a predetermined frequency, b. measuring the responseof the instrument as tuned in step (a) to a continuous wave signal ofpredetermined amplitude whose frequency is the same as saidpredetermined frequency, and c. varying the center frequency of saidinstrument over the frequency range of interest in order to determineits response to a white noise signal of uniform amplitude throughoutsaid frequency range of interest, after first adjusting the amplitude ofsaid white noise signal so that the measurement indicated by saidinstrument while it is tuned to said predetermined frequency is the sameas the measurement indicated in step (b).
 3. A system for determiningthe absolute response of a device under test throughout a frequencyrange of interest comprising a tunable instrument for measuring andindicating the power in a fixed bandwidth whose center frequency may bevaried, and signal generating means, the device under test beingconnected between said signal generating means and said instrument suchthat said instrument measures and indicates the response of said deviceunder test to The signal furnished thereto by said signal generatingmeans, said signal generating means including means for generating acontinuous wave signal of predetermined amplitude and frequency, whitenoise signal generating means, means for selectively applying one ofsaid continuous wave and white noise signals to said device under test,and means for adjusting the amplitude of said white noise signal suchthat said instrument indicates the same measurement while it is tuned tosaid predetermined frequency when each of said continuous wave and whitenoise signals is applied to said device under test.
 4. A system fordetermining the absolute response of a tunable instrument throughout afrequency range of interest, said instrument being operative to measureand indicate the power in a fixed bandwidth whose center frequency maybe varied, comprising signal generating means for connection to saidinstrument such that said instrument indicates its response to thesignal applied thereto by said signal generating means, said signalgenerating means including means for generating a continuous wave signalof predetermined amplitude and frequency, white noise signal generatingmeans, means for selectively applying one of said continuous wave andwhite noise signals to said instrument, and means for adjusting theamplitude of said white noise signal such that said instrument indicatesthe same measurement while it is tuned to said predetermined frequencywhen each of said continuous wave and white noise signals is appliedthereto.